The Essential Oil of Hops: Hop Aroma and Flavor in Hops and Beer

The female flower of the hop plant (Humulus Lupulus) has long been a friend to beer
brewers around the world. Depending on the style of beer, hops can add balance to the
finished product through bitterness, flavor, and aroma. It is well documented how
bitterness is obtained in the finished product via the isomerization of alpha acids, but
investigations into the chemistry and utilization of hop essential oils for beer flavor and
aroma have, in general, failed to elucidate the complex processes involved. In this article
I begin with some background on hops and their use in brewing beer, and then narrow
the focus to examine the essential oil of hops. I first discuss the chemistry and
composition of hop oil in hops and beer, and then quickly turn to the practical by
discussing how to evaluate and analyze hops for the aromatic quality of hop essential
oil. I conclude with some methods to obtain and protect hop flavor and aroma in your
finished beer, which I think is the most important take home message of all.

As dedicated small scale brewers, you are more than likely familiar with the wide variety
of hop products available for use in brewing. The starting point for all of these is the
whole unprocessed hop flower. Hop flowers are harvested once a year, processed,
packaged, and held in cold storage until sold. Depending on the level and intensity of
processing, many different products can be produced including Type 100 pellets (highly
compressed whole cones in half ounce plugs), Type 90 pellets (ground whole cones
extruded into pellets), and a wide variety of purified and concentrated extracts and
essences. In most cases, there are trade-offs, especially with respect to hop oils, that
roughly scale with the intensity of the processing. Each of these products has a slightly
different contribution to beer, especially when you consider the wide variety of ways
that brewers have come up with to introduce hop character into their product.

By far the most common way that hops are used is to add them to the brewkettle and
boil them in the sweet wort. For long boils (30-120 minutes) hops accomplish four
major things: they add bitterness via alpha acids being isomerized (rearranged without
changing their composition) into more stable and soluble iso-alpha acids, they assist in
the production of a good hot break by supplying tannins that combine with unwanted
proteins, they add to beer stability due to their antibacterial properties, and they lower
the surface tension of the wort so that a vigorous boil can more easily be maintained.
Late kettle additions (0-30 minutes before strikeout) are responsible for most of the
"hop character" of a beer, i.e. hop flavor and aroma from the essential oils of the hop.
Running hot wort through a bed of hops on the way to the chiller (i.e. using a hop back
or jack) can introduce even more hop oil to the wort. Dry hopping, i.e. adding hops to
the fermenter or serving tank, is another method that accomplishes this although the
compounds extracted by cool wort or beer are quite different than those found in a
wort after the boil.

The various extract and essences available give the brewer even more choices of when
to add hop compounds. Some British brewers have abandoned dryhopping in favor of
post-fermentation additions of concentrated hop oil. Pure iso-alpha extracts give equal
flexibility in the fine tuning of beer bitterness. To put this all in a nut shell, hop character
is affected both by what type of hop product you add to the wort or beer, and by the
method and timing of the addition. In part this is due to the vast array of very reactive
compounds that make up hop oil.

There are two groups of oils found in most plant materials, differentiated by their
volatility. The fixed oils, like liquid fats found in nuts, are relatively nonvolatile and do
not contribute much to aroma. The volatile oils, also called the essential oils, are so
easily vaporizable that we can deduce their presence or absence by simply using our
noses. We are all familiar with perfumes and spices, both of which are loaded with
either naturally occurring or synthesized essential oils. In hops, the essential oil makes
up only about 0.5-3% (vol/wt) of the whole cone, but the contribution to beer is
enormous. Think about it, if a typical beer uses hops at a rate of 0.5 ounces per gallon,
and we assume that the oils are 10% utilized, then the hop oils end up being less than
0.001% wt/wt (10 ppm) of the finished beer. And yet what is the first thing that hits the
nose in a Cascade dryhopped American pale ale? That's right, a big floral aroma that is
impossible to miss. To examine just what it is in the hop essential oil that makes such big
impressions let's talk chemistry.

Hop oils are made up primarily of a hydrocarbon fraction, (containing only H and C),
and an oxygenated fraction (H, C and O). There are also small amounts of sulfur
containing compounds but we will neglect them in this context. The hydrocarbons
typically make up 80-90% of the total oil; the terpenes myrcene and b-pinene, and the
sesquiterpenes b-caryophyllene and a-humulene are found in the largest quantities. Two
of these, b-caryophyllene and a-humulene, can be easily oxidized in air, thus
contributing to the oxygenated fraction of the oil as well. Other oxygenated compounds
include alcohols such as linalool and geraniol and esters such as geranyl isobutyrate and
methyl dec-4-enoate. Although many brewers think esters are all fermentation by-
products, hops can contribute a number of fruity aromas, grapefruit and pineapple for
example.

Some of you may have already asked the question, "Why hasn't anyone tried to quantify
the 'hoppiness' of a given hop sample like they have its bitterness (i.e. alpha acid %)?"
Since hop character is attributed to hop essential oil, one would think that the amount
of oil in a hop sample would be a way to estimate its hop aroma content. Unfortunately
things are not so simple. The total oil content can be an indicator of the overall quality
of a hop sample, especially if you have a good idea what the normal oil content for that
hop should be. But because there are many factors that affect essential oil production
and preservation, every season there is potential for significant variation in both the
total amount of oil and the composition of the oil even for a single variety. In fact from
farm to farm there can be substantial differences, not to mention the additional
variables introduced by the hop processing and storage facilities. Over the period 1975-
1985 the total oil content of fresh Cascade hop samples at the U.S.D.A. hop lab varied
between 0.28 and 1.79 ml/100 g of hops and the percent myrcene, usually the the
major component in the oil, ranged from 46% to 82%. Since both the total oil content
and the relative amounts of hop oil components can vary widely, it would follow that a
simple oil content rating would not reliably provide the information we need to evaluate
the aroma quality of our hops. There are however three other methods that have all
proven to be quite helpful.

First I will outline the high tech method which gives both quantitative and qualitative
information but requires distillation equipment and a capillary gas chromatograph (GC).
The GC has helped researchers to identify more than 250 essential oil components. Of
these, 22 that have been reported to affect hop aroma in the literature have been
divided into groups: Humulene and Caryophyllene Oxidation products, Floral-Estery
Compounds, and Citrus-Piney Compounds. These 22 compounds make up the Hop
Aroma Component Profile (HACP). Just as bitterness in beer is quantified using the
Bittering Unit (BU), Nickerson and Van Engel propose that an Aroma Unit (AU) be
adopted which is defined as 1 ppm (Ál/kg of hops or Ál/l of wort or beer) of the sum of
the 22 HACP constituents. Thus if the AUs of a brewery's aroma hops changed, it would
be a simple matter to calculate the change in the amount of hops needed to achieve the
historical AUs of a given beer. How well do AUs correlate with actual 'hoppiness'? A
commercial brewery's taste panel found that hop aroma, hop taste, and dry hop aroma
correlated very well with AU's.

Using data from an earlier study, I checked to make sure that nothing more simple was
being overlooked, since as a homebrewer I didn't happen to have a spare GC laying
around my basement. Although there was a correlation between the AU's and the total
oil content in fresh hops it was "weak" (r = .449), ranking only about a 3 on a scale of
one to ten. The correlation was better in hops aged for six months at room temperature
(r = .756) but hopefully none of us are brewing with hops that have been subject to
such storage conditions. This confirms our previous hypothesis that total oil content is
not necessarily a good indicator of potential hoppiness or aroma quality. There were
also no significant relationships between myrcene, humulene, or alpha acid levels and
AUs.

For those of us without a complete lab at our disposals, here are a couple of tried and
true low tech (no tech!?) methods for evaluating the aroma potential and quality of
hops). The first is initiated by grabbing a handful of hops and looking/listening/feeling
/tasting/smelling them. Jean De Clerck wrote in reference to evaluating fine aroma hops,

"Aroma is tested by smelling a crushed handful of whole cones. The hops should have a
pronounced aromatic smell free from extraneous taints and odors, [list of unpleasant
odors]. Smell may also be tested by rubbing the cones between the fingers which splits
the lupulin grains. The aroma of the sample should not be sharp, but fine and
mellow."

One reminder however, is that although this sort of manual evaluation of hops will give
you a good feel for the over-all quality of the hop and its potential dryhop aroma, it
provides little quantitative information. Most of what you will smell is the hydrocarbon
fraction of the essential oil which is rapidly lost during the boil and/or changed during
fermentation and is, in fact, rarely found in beer. If after crushing the hops in your
hands you pause for a minute or two to let the most volatile of the hydrocarbons
dissipate, you will be better able to evaluate the aroma quality.

The second method is to make a hop tea by boiling or steeping a hop sample in water.
This evaporates a lot of the hydrocarbons and gives you a better impression of how a
particular sample will contribute in the brewkettle. In order to be as consistent as
possible, I recommend always using the same container to make the tea (a 1 L
erlenmyer flask is good) and the same water to hop ratio (2 g hops per 600 mL water).
Prepare the tea the same way you plan to use the hops in your beer, boiling (or
steeping) for a predetermined time. After cooling the tea add water to bring the volume
back to your starting point and evaluate the flavor and aroma both for quality and
quantity/intensity. Although this method does not employ any fancy instrumentation, it
can be very effective, especially if good sampling notes are kept. One final thing to
remember with respect to both of the low tech methods: if it smells bad, don't use it!

Hop essential oils, like hop bitter resins, are easily lost during storage due to their
susceptibility to oxidation. In storage trials at the U.S.D.A. Hop Research Lab at O.S.U. in
Corvallis, OR , oil losses were found to range from 28-90% in six months at room
temperature depending on the variety. These losses can be slowed by storing hops in
the freezer, preferably in a package that allows no air or water exchange (i.e. "barrier").
The best advice to follow in this area is to buy the best hops you can find and take good
care of them, especially if they are aroma hops.

As is mentioned above, the compounds found in hop oil are not all the same as those
found in hopped beer. Late kettle hops contribute differently than steeped hops and dry
hops as most of you already know. Both the fermentation and finishing processes also
affect hop character. Consequently a brewer needs to find the type of hop character
s/he desires in a given beer and then be consistent in the method used to brew it. Then
once you have achieved the Holy Grail of hoppiness, and the bottle is capped or the keg
is sealed, hop character starts to go downhill; unfortunately it is inevitable. I'll tell you
how this occurs and some steps to take to prevent it.

Peacock and Deinzer found that the two main ways in which hop oil components in
beer can be lost are migration into the packaging material and chemical degradation.
They write,

"The major mechanism for the loss of hop aroma compounds from beer
appears to be chemical degradation . The most likely pathways for degradation of
terpenoid and sesquiterpenoid compounds are reaction with oxygen in the headspace
of the bottle and acid hydrolysis."

They go on to say that bottle cap liners tend to absorb many hop compounds, especially
hydrocarbons and longer chain ketones and esters. Hop oil alcohols appear to not be
absorbed well by the cap liners. Since we can not do anything about acid hydrolysis, this
diagnosis points to a rigorous elimination of oxygen from all beer packages, and to using
a non-absorptive bottle cap liner. Oxygen scavenger caps (PureSeal) are on the market
now but their performance with respect to absorption of hop oil compounds has not
yet been tested.

We've covered a lot of ground here and you may be wondering what the take home
message is to all of this. I think there are four important points to remember when
dealing with the essential oils in hops.

Start with the best hops you can get and store them in the freezer in barrier
packaging if possible.

Monitor the aroma quality of your hops using either the Aroma Unit (if you can)
or by physical evaluation.

Be very consistent in your brewing process making adjustments based on your
evaluations.

Try to eliminate as much oxygen as possible from your packaging process.

Steps 3 and 4 will yield benefits in other areas, so the pay-off is even bigger.

Hop oil research is a rapidly changing field and new findings pop up every year. The
consensus is that the marvelous hop aroma and flavor we all enjoy is probably due to a
synergistic combination of hop oil compounds and their oxidation and fermentation
products, some of which may have not even been discovered yet. All I can say is that
even though I may not know exactly what it is I am tasting in my dryhopped IPA, I like
it!